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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<!-- $Id$ -->
<html> <head>
<title>Using ACE try Macros to Enhance CORBA Portability</title>
</head>
<body text="#000000" link="#0000ff" vlink="#cc0000" bgcolor="#ffffff">
<Hr><P>
<h3>Using ACE try Macros to Enhance CORBA Portability</h3>
<P>CORBA <CODE>Environment</CODE> arguments provide a way to handle
exceptions when native c++ exception handling is unavailable or
undesirable. However, writing portable code using both native C++
exception handling and <CODE>CORBA::Environment</CODE> objects is very
hard. If you plan to write portable code that must run on platforms
that do not have native C++ exceptions, therefore, we recommend you
use the ACE exception macros. This document explains how these macros
can help alleviate much of the accidental complexity. However, keep
in mind macros cannot solve all problems perfectly. </P>
<P>Before reading the rest of this document, we recommend you check
out pages 307 through to 322 in the book, <A
HREF="http://cseng.aw.com/bookdetail.qry?ISBN=0-201-37927-9&ptype=0">Advanced
Corba Programming with C++</A> by <A
HREF="http://www.triodia.com/staff/michi-henning.html">Michi
Henning</A> & <A HREF="http://www.iona.com/hyplan/vinoski/">Steve
Vinoski</A>. Likewise, we recommend that you read chapter 6 from the
<A
HREF="http://www.theaceorb.com/TAO/Purchase_TAO/purchase_tao.html">TAO
Developer's Guide</A>.
<P><HR><P>
<h3>Table of Contents</h3>
<ul>
<li><a href="#nutshell">ACE Try Macros in a Nutshell</a>
<li><a href="#examples">Examples</a>
<li><a href="#general">General Guidelines for Exception Handling</a>
<li><a href="#transition">Transition from TAO try macros to ACE try
macros</a>
<li><a href="#caveats">Some Caveats</a>
</ul>
<HR><P>
<h3><a name="nutshell">ACE Try Macros in a Nutshell</h3>
<P>This section explains some simple rules of writing programs for
platforms with and without native exception support using ACE's
try macros.
</P>
<ol>
<li><p>ACE try macros are modelled like C++ language exceptions and
can be used like them, but with a small difference. These macros
rely on the CORBA::Environment variable to handle exceptions
on platforms that do not support exception
handling. (Please note that native exceptions can be turned on
or off at COMPILE time as an option to your make)
The exception macros have been modelled with some extra rules to ensure it works even on
platforms without native exception support. See some <a
href="#examples">quick examples</a> on how to use ACE try
macros.
</p>
</li>
<li><P><em>Name of CORBA::Environment variable</em><br>
A function that may throw a CORBA::Exception needs a
CORBA::Environment variable to pass up exceptions (to throw in
the C++ sense) and to gather (catch () in the C++ sense)
exceptions from functions it called. By default, ACE try macros
assumes that the variable is named <code>ACE_TRY_ENV</code>.
<code>ACE_TRY_ENV</code> itself is also a macro which can be
redefined.
</pre>
<P>
You can redefine the name of the variable to
something else to avoid name clashing. Alternatively, there's
another macro (<code>ACE_ADOPT_CORBA_ENV</code>) that allow you
to use another variable name as the default CORBA::Environment
<em>within</em> a function.
</P>
</LI>
<li><P><em>Definition of the CORBA::Environment variable </em><br>
If you are using TAO for writing application programs and you
are interested in using exceptions (which is a pretty neat way
to go about), the environmnet variable can be brought in to your
scope by the adding a statement <code>ACE_DECLARE_NEW_CORBA_ENV;</code>
You can then invoke the methods on the servant from the client
side as
<pre>
object_reference->func_name (x, y, ACE_TRY_ENV);
</pre>
Even if you are interested in making calls within the client
side, you can define your method like this
<pre>
int AN_OBJ::foobar (int a, int b, CORBA_Environment &ACE_TRY_ENV);
</pre>
<li><P><em>Throwing exceptions:</em><br>
Use <code>ACE_THROW</code> and <code>ACE_THROW_RETURN</code> to
throw exceptions. They should never be used within a try
block; please use <code>ACE_TRY_THROW</code> instead.
</P>
</LI>
<li><P><em>Propagating exceptions:</em><br>
To simulate native exceptions on platforms without native
exception handling, <em>every</em> function call that may
throw exceptions must be followed by <code>ACE_CHECK</code> or
<code>ACE_CHECK_RETURN</code>.</p>
<p><a name="exc-func">Exception-throwing functions include the
following categories:</p>
<ol>
<li><p>Any function that takes a
<code>CORBA_Environment</code> argument.</p>
</li>
<li><p><code>ACE_NEW_THROW_EX</code>. Notice that you
<em>should not</em> use <code>ACE_NEW_THROW</code>,
<code>ACE_NEW_THROW_RETURN</code>,
<code>ACE_NEW_TRY_THROW</code> anymore because they don't
work right with ACE try macros. Instead, use
<code>ACE_NEW_THROW</code> with appropriate ACE_CHECK*
macros.</p>
</li>
<li><P><code>ACE_GUARD_THROW_EX</code>,
<code>ACE_READ_GURAD_THROW_EX</code>, and
<code>ACE_WRITE_THROW_EX</code>.
<li><p><code>ACE_TRY</code> blocks. Follow every
<code>ACE_ENDTRY</code> with appropriate ACE_CHECK*
macros.</p>
<li>
</ol>
<P>You should pass <code>ACE_TRY_ENV</code> to these
functions.
</p>
<P>Be very careful not to combine exception throwing functions
in one statement like this:
</P>
<pre>
x = obj1->callme (ACE_TRY_ENV) + obj2->dare_me (ACE_TRY_ENV);
ACE_CHECK;
</pre>
<P>This example may work differently when native exception
handling is enabled/disabled.
</p>
</LI>
<li><P><em>Catching exceptions:</em><br>
Use <code>ACE_TRY</code> to catch exceptions if there's an
<code>ACE_TRY_ENV</code> available. Otherwise, you should use
<code>ACE_DECLARE_NEW_CORBA_ENV</code> to create one at
<em>proper</em> scope. The use of
<code>ACE_TRY_NEW_ENV</code> is considered depricated because it
can't deal with the case when you have multiple <code>TRY</code>
blocks in the scope of <code>ACE_TRY_NEW_ENV</code>. If there are
more than one try blocks in a function, use <code>ACE_TRY_EX</code>
for all subsequence try blocks to avoid name clashing of labels.
</p>
<ul>
<li><P>Within a <code>ACE_TRY</code> block, use the variable
<code>ACE_TRY_ENV</code> to pass down the
<code>CORBA_Environment</code> (see <a
href="#try_env">this</a> example.)
</p>
</LI>
<li><P>Follow <em>every</em> exception throwing function with
<code>ACE_TRY_CHECK</code>. If you are using a TRY block
within another try block add a <code>ACE_TRY_CHECK</code>
at the end of this TRY block ie. after
<code>ACE_ENDTRY</code>.
</p>
</LI>
<li><P>Use <code>ACE_CATCH</code> to catch exceptions of certain
type.
</p>
</LI>
<li><P><code>ACE_CATCHANY</code> catches <em>any</em> exceptions
of type <code>CORBA_Exception</code>. The caught
exception is stored in a variable call
<code>ACE_ANY_EXCEPTION</code>.
</p>
</LI>
<li><p><code>ACE_CATCHALL</code> emulate the <code>catch
(...)</code> c++ statement. It is identical to
<code>ACE_CATCHANY</code> on platforms without native
exception support. You can not access the caught
exception within the <code>ACE_CATCHALL</code> block.</p>
<li><P>Use <code>ACE_RETHROW</code> to rethrow the same exception
within a <code>ACE_CATCH</code> or
<code>ACE_CATCHANY</code> block.
</p>
</LI>
<li><P>A <code>ACE_TRY</code> block must be terminated with
a <code>ACE_ENDTRY</code> statement.
</p>
</LI>
<li><P>Throw an exception within a <code>ACE_TRY</code>
block or <code>ACE_CATCH</code> block using
<a href="#ace_try"><code>ACE_TRY_THROW</code></a>.
</p>
</LI>
</ul>
</li>
<li><p><em>Printing out exceptions.</em> Use <code>ACE_PRINT_EXCEPTION
(EX,INFO)</code> to print out an exception. The macro takes two
arguments, a reference to an exception (EX) and a <code>char
*</code> string (INFO) which provides more information on the
exception. Since there's no portable way to print out
exceptions, you can redefine ACE_PRINT_EXCEPTION to fit your
need (or define it to null.) <em>You should always print out
the exception itself, not the CORBA_Environment that carries the
exception.</em></p>
</li>
</ol>
<HR><P>
<h3>Examples</h3><a name="examples">
Refer to <a href="../ace/CORBA_macros.h"><code>
$ACE_ROOT/ace/CORBA_macros.h</code></a> for complete definitions of
macros discussed here.
<ul>Examples on using ACE try macros:
<li><p>
<pre>
<a name="try_env">
ACE_TRY // Use ACE_DECLARE_NEW_CORBA_ENV to create ACE_TRY_ENV
// if you got undefined symbol warning here.
{
some_operation (arg1, arg2, ACE_TRY_ENV);
ACE_TRY_CHECK;
.
.
if (whatever)
ACE_TRY_THROW (CORBA::BAD_PARAM ());
some_other_operation (arg1, arg2, arg3, ACE_TRY_ENV);
ACE_TRY_CHECK;
}
<a name="ace_try">
ACE_CATCH (CORBA_some_exception, ex)
{
// error handling.
if (still_has_error)
ACE_TRY_THROW (CORBA::NOWAY ());
}
ACE_CATCHANY
{
// error handling.
// then rethow the exception.
ACE_RETHROW;
}
ACE_ENDTRY;
ACE_CHECK;
</pre>
</p>
</li>
<li><p><code>ACE_TRY</code> and also declares a label for internal
use. To avoid defining the same label multiple times within a
function, use <code>ACE_TRY_EX</code> with different labels for
different try blocks instead. For example,<br>
<pre>
ACE_TRY_EX (block_1)
{
some_operation (arg1, arg2, ACE_TRY_ENV);
ACE_TRY_CHECK_EX (block_1);
some_other_operation (arg1, arg2, arg3, ACE_TRY_ENV);
ACE_TRY_CHECK_EX (block_1);
}
ACE_CATCH (CORBA_some_exception, ex)
{
// error handling.
}
ACE_CATCHANY
{
// error handling.
}
ACE_ENDTRY;
ACE_CHECK_RETURN (-1);
// Some other operation here
// .
// .
// .
// .
ACE_TRY_EX (block_2)
{
foo (arg, ACE_TRY_ENV);
ACE_TRY_CHECK_EX (block_2);
bar (arg1, arg2, ACE_TRY_ENV);
ACE_TRY_CHECK_EX (block_2);
}
ACE_CATCH (CORBA_some_exception, ex)
{
// error handling.
}
ACE_CATCHANY
{
// error handling.
}
ACE_ENDTRY;
ACE_CHECK_RETURN (-1);
</pre>
</p>
<li><p>You may want to make a different series of calls after you
encounter/catch an exception. Here is what we recommend.
<pre>
ACE_TRY
{
// Calls that can raise an exception
some_call1 (arg1, arg2, ACE_TRY_ENV);
ACE_TRY_CHECK;
.
.
.
ACE_TRY_CHECK;
}
ACE_CATCH (CORBA_some_exception, ex)
{
// Caught an exception, so we need to make some other calls
// to continue..
ACE_TRY_EX (block1) // basically a label
{
some_other_call1 (arg1,.. , ACE_TRY_ENV);
ACE_TRY_CHECK_EX (block1);
}
ACE_CATCH (CORBA_some_other_exception, ex1)
{
// Handle the exception here..
}
ACE_ENDTRY;
ACE_CHECK_RETURN (-1); // Needed to catch uncaught exceptions
}
ACE_ENDTRY;
ACE_CHECK_RETURN (-1);
</pre>
</p>
<li><p>Be <em>VERY</em> wary of <code>ACE_CATCHALL</code>. It catches
exceptions of any type. If your program depends on it, then,
more often than not, there're something wrong with it.
</P>
</li>
<li><p>Instead of depending on <code>ACE_CATCHALL</code>, use
<code>auto_ptr</code> style mechanism to prevent memory leaks
from exceptions.
</p>
</li>
</ul>
<HR><P>
<h3><a name="general">General Guidelines for Exception Handling</h3>
<ul>
<li><p>Don't catch an exception just to rethrow it. Exceptions cost
you performance.
</p>
</li>
<li><p>When exceptions occur, make sure an object's is still in
a valid state or change to a state that can be safely
destructed.
</p>
</li>
<li><p>Watch out for side effect in the expression which may cause
exceptions. In the following example, what should
<code>i</code> be if an exception does occur?<br>
<pre>
ACE_TRY
{
obj[i++] = foo_bar_method (a, b, ACE_TRY_ENV);
}
</pre></p>
</li>
<li><p>Make sure an exception doesn't cause resource leak (memory,
socket, ...) (hint: Use auto_ptr to avoid memory leak,
and ACE_Guard for locks.)
</p>
</li>
<li><p>Don't catch any exception that you don't know how to handle.</p>
</li>
<li><p>Never throw an exception from destructor (unless you know what
it implies.)</p>
</li>
<li><p>Use exceptions to provide more information about the error.</p>
</li>
<li><p>Rethrow a different exception only to provide <em>more</em>
information. Do not catch an exception just to rethrow, say,
<code>unknow_exception</code>.</p>
</li>
</ul>
<HR><P>
<H3><a name="transition">Transition from TAO try macros to ACE try macros</h3>
This list tries to give a comprehensive list of mapping between TAO
try macros and ACE try macros. It's sole purpose is to provide hints
in the converting the use of TAO try macros to ACE try macros and is
by no mean complete.
<OL>
<li><P>Rename all <code>CORBA_Environment</code> variables to
<code>ACE_TRY_ENV</code>. </P>
</li>
<li><P>Replace <code>TAO_TRY</code> <code>TAO_TRY_VAR</code> with
<code>ACE_TRY</code>. Added
<code>ACE_DECLARE_NEW_CORBA_ENV</code> if necessary.</p>
</li>
<li><p>Replace <code>TAO_TRY_EX</code> <code>TAO_TRY_VAR_EX</code>
with <code>ACE_TRY_EX</code>.</p>
</li>
<li><p>Replace <code>TAO_CHECK_RETURN</code> and
<code>TAO_CHECK_RETURN_VOID</code> with <code>ACE_CHECK_RETURN</code>
and <code>ACE_CHECK</code>. These macros are used
<em>outside</em> of TRY/CATCH blocks.</P>
</li>
<li><P>Replace <code>TAO_THROW</code>, <code>TAO_THROW_ENV</code>,
<code>TAO_THROW_RETURN</code>, <code>TAO_THROW_ENV_RETURN</code>
with <code>ACE_THROW</code> and
<code>ACE_THROW_RETURN</code>. </p>
</li>
<li><P>Replace <code>ACE_NEW_THROW</code>,
<code>ACE_NEW_THROW_RETURN</code>,
<code>ACE_TRY_NEW_THROW</code> with
<code>ACE_NEW_THROW_EX</code> and appropriate
<code>ACE_CHECK*</code> macros. aformention</P>
<li><p>Replace <code>TAO_CHECK_ENV</code> and
<code>TAO_CHECK_ENV_EX</code> with <code>ACE_TRY_CHECK</code>
and <code>ACE_TRY_CHECK_EX</code>.</p>
</li>
<li><p>Replace <code>TAO_TRY_THOW</code> and
<code>TAO_TRY_THROW_EX</code> with <code>ACE_TRY_THROW</code>
and <code>ACE_TRY_THROW_EX</code>. Notice that you can also use
<code>ACE_TRY_THROW*</code> within CATCH blocks.</p>
</li>
<li><p>Replace <code>TAO_RETHROW</code>,
<code>TAO_RETHROW_RETURN</code>,
<code>TAO_RETHROW_RETURN_VOID</code> with
<code>ACE_RETHROW</code>.</p>
</li>
<li><p>Replace <code>TAO_CATCH</code>, <code>TAO_CATCHANY</code>,
and <code>TAO_CATCHALL</code> with <code>ACE_CATCH</code>,
<code>ACE_CATCHANY</code> and <code>ACE_CATCHALL</code>
respectively. </p>
</li>
<li><p>Replace <code>TAO_ENDTRY</code> with <code>ACE_ENDTRY</code>
followed by an appropriate CHECK macro (<code>ACE_CHECK</code>,
<code>ACE_CHECK_RETURN</code>, <code>ACE_TRY_CHECK</code>, or
<code>ACE_TRY_CHECK_EX</code>.)</P>
</li>
</ol>
<HR><P>
<H3><a name="caveats">Caveats</H3>
<P>As we already mentioned no set of macros can cover all cases
and preserve the semantics between native C++ exceptions and the
<CODE>CORBA::Environment</CODE> based mapping.
Some of the problems that our macros are described below:
<P>
<UL>
<LI><P>Using the macros in loops can produce problems with
<CODE>break</CODE> and <CODE>continue</CODE> statements, for
example:
</P>
<PRE>
for (int i = 0; i < 10; ++i)
{
ACE_TRY
{
if (x[i] == 0)
continue; // will *not* work
if (x[i] == -1)
break; // will *not* work either
}
ACE_CATCH (CORBA::Exception, ex)
{
}
ACE_ENDTRY;
ACE_CHECK;
}
</PRE>
</LI>
</UL>
<P><HR><P>
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HREF="http://www.cs.wustl.edu/~schmidt/ACE-documentation.html">ACE
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